Means for controlling flow of electrolyte to an electrolytic cell



M y 11, 1954 J. M. STURTEVANT 2,678,300

MEANS FOR CONTROLLING FLOW OF ELECTROLYTE TO AN ELECTROLYTIC CELL Filed Nov. 7, 1946 al/i022 722. Jigt feufifii fi z M Patented May 11, 1954 IVIEANS FOR CONTROLLING FLOW OF ELEC- TROLYTE TO AN ELECTROLYTIC CELL Julian M. Sturtevant, Branford, Conm, assignor to the United States of America, as represented by the United States Atomic Energy Commission Application November 7, 1946, Serial No. 708,426

5 Claims. 1

This invention relates broadly to a method and apparatus for automatically titrating a fluid, mainly for maintaining constant the position of aliquid-liquid boundary formed by superimposed fluids. More specifically the present invention relates to an improved method and apparatus for controlling the feed of a liquid electrolyte in an electrolytic isotope separating apparatus of. the type described and claimed in the copending application of Samuel L. Madorsky et al., S. N. 432,185, filed February 2, 1942, now U. S. Patent No. 2,645,610, issued July 14, 1953.

In the method of electrolytic isotope separation as described in the above identified copending application the electrolytic or reaction bath comprises a solution of a salt of a metal the isotopes of which are to be separated. This salt solution is situated in an electrolysis cell or chamber provided with a permeable barrier positioned between the electrodes. To effect a separation of the isotopes ofthe desired material a voltage is applied between the electrodes, thereby imparting velocities to the cations in accordance with their respective masses. An opposite drift or counterflow is imparted to the entire solution by slowly titrating an electrolyte at the oathode end of the cell. If the rate of feed of this electrolyte is properly adjusted with respect to the current and cation drift only the lighter or faster moving cations will reach the vicinity of the cathode.

The'present invention is directed to a method and apparatus for automatically maintaining the feeder" this electrolyte, to effectively retard the less mobile cations and thereby insure proper operation of the separator.

In operation of such electrolysis cells, a liquidliquid boundary has been observed (due to the different indicesof refraction of the two liquids) between the electrolyte and the salt solution of the desired material (the position of this boundary being dependent upon the acid concentration of the electrolyte adjacent to the cathode). As the electrolysis proceeds the motion of the ions in the electric field causes this boundry to move. For proper operation the liquid electrolyte should be added so as to maintain this boundary stationary.

The operation of my improved method and apparatus for controlling the feed of this electrolyte is predicated on the observation that the electrical conductivity of the electrolyte changes rapidly at the boundary (the conductance of the acid electrolyte being greater than the conductance of the salt solution); and upon my discovery that the temperature of the poorly electrically conducting solution is higher than the more readily electrically conducting solution as a result of the flow of the electrolysis current. Coincident with this discovery I hit upon the idea of utilizing this temperature difierence in the two liquids as a means for controlling the titration or feed of the simple liquid electrolyte to maintain the boundary layer substantially stationary throughout the operation of the electrolysis cell. It occurred to me that a resistance unit inserted in the cell solution with the boundary located at its midsection for example, would have a temperature that would vary with any change in the location of the boundary layer;

and that this change in temperature would alter the electrical resistance of this unit. Accordingly I set out to devise a system for controlling rate of feed of the liquid electrolyte that would be responsive to changes in the boundary position whereby the rate of feed would be adjusted automatically to maintain a predetermined acid concentration at the cathode thereby keeping the boundary fixed in position.

In a preferred embodiment of this invention I utilized an extremely fine enameled copper wire encased in a Pyrex tube as the resistance unit; and inserted this resistor in the electrolytic solution adjacent to the cathode with the boundary temperature concomitant with the movement of boundary layer of the salt solution and liquid acid electrolyte for controlling the feed of acid in a manner to counteract the'aforesaid movement of the boundary.

I prefer to utilize the amplified output from the bridge network to heat a bi-metallic strip upon which is supported a levelling cup containing the. said liquid electrolyte at a constant predetermined level therein; and to utilize a flexible conduit to permit the fiow of liquid from the cup into the reaction chamber or, electrolysis cell.

With this arrangement the pressure head of the liquid electrolyte in the cup is altered through the flexing of the bimetallic strip to control the ling the feed of one of said -liquidsiinzamanner'r to counteract the aforesaid movement of said boundary.

A more limited and specific object of this'invention to provide an improved methodres-:1:

controlling the counterfiow in an electrolytic system forth'e separation of isotopes -.to the-end that the less mobile-"ions will ,be-effectivelyretarded and the more mobile ions permittedto? reach the cathode region. Suohrautcmaticmon trol of drift or counteriiowrmalies it'-possible.-:for-

one attendant to supervise the operationmf a large number ofseparators; :but what is: more important such automaticcontroli improves the-: efficiency of the isotope separating system.

In the drawings 3 Fig.1 is a=diagrammatic View partly in" cross sectionof an electrolytic systemior separation of isotopes in which my invention has particular application; and Fig'.';2 :is'a schematic-illustras tio-n of the apparatus-employedin practicingthe invention;

As is'shownin Fig.1 an apparatus for-the separation of" isotopes a ofnax substance ma-y include a I" U-shaped glass .tube- 1 0; provided' at one end with an enlarged cup-shaped portion I H into which an anode l2 is-inserted; A spillway I3 is-fo'rmed in .the Wall of the tube at the anode region; I l:.

Similarly, the 'otheri-end of .-:the "U-shaped tube I0 is enlarged to provide-a cup-eshapedcathodefi region E i into which thecathodewl'! is inserte'd. 4

Disposedin' the tube ii} in'xthebottom ofathe. Us shaped portion; for example, and intermediate between thecathode "and the'anode; is'a permee ableplug IE ofalu'ndum or the like." A'glass tube I a is connectedinto'the ueshapedztube lfl atr the cathode region" 14' to provide a br'ancharmthereof.

It will be-understood that" the apparatus may be used 4 for. the separation": ofisotopesv'of "any. T

metal of which the:ionized-isotopes"possess dif-v ferent mobility.-= By Way" of example uranium. 4

has isotopes U and U the ion otU being- 11 more mobile in anelectrolyte than the -i01ll'0f U3 In carrying out the separation 0117 and 113 a solutionnf. UOzChmay, be .fedinto .thenglasss tube It ata predeterminedrateby meansof the; feed tube 313 fpositioned'at the anode end there-. of. Thefluid in excess of the required quantity. flows out of the tube lfl'by means of the spill- Way 3.. An HCl solution is added incontrolled amounts at the. cathode end of the tube ill in. a mannerfully explained below? The functionof the 'alundum'plug lfi" is to minimize remixing by turbulence and convection? The plug'perin'its the flow'of ions but also quiets'7g the now through the separating zones The lighter U ions, of higher "mobility-, would tend to accumulate around the "cathode at a faster 1' rate than the less mobile isotope ionsg'i. 'e.,-:those.

taken and time is allowed to elapse, both isotopes Will eventually gather at the cathode 11. However, by adding a liquid electrolyte at the cathode to maintain a sufiicient number of ions (other than uranium ions) to insure the desired electrolyticflow of current between the-working electrodes, the. resulting flow of: liquid' will lo-ppose the electrolytic migration of uranium ions towards the cathode; and if the rate of flow of the added electrolyte is appropriately balanced againstr'uraniumion migration the uranium ions can'bejinefie'ct; selectively retarded in an inverse-wrelati'on-totheir mobilities. Thus in a system: off-the type shown, by the addition of H01 (i;e., arr-aqueous solution of hydrochloric acid) at the cathode region at a carefully determined rate,.-the-.heavy 1733 ions will tend to be swept back more than the lighter and more mobile U -*ions or rather the latter ions will be more able to overcome the counterfiow and will progress furtherrtoward thecathode 1 l. Under presently preferred conditions, :theyflow isadjusted: so. 1 that-:fewpxif any. of; the uranium ionsuactually: reachthecathode, and a so-calledyboundary-layer betweenrthe H531v sandrnozon. sclutionsiiis thus formed; (provided the :addition: of- HG} isr-at: the-. properrate) at aposition in the system rbetween the cathodexand the 'plug. lBye; g.,. as indicated.

by'the referencenumeral. 29; Under. proper.- con-.. ditions thisboundary layerxcan actuallyber-zseen because-of the difference: in. index offirefraction of the two solutions: The.-U 'ions:.-tend tormie. grate to the region. of' 'thisrzboundaryzwhile? the counterfiow retards 1 the :U ions .1 to. ::a sgreaters" "extent soithat a; gradient of the :l.(-BIZJblVE"iCOll.-'

centration- 01717 3 to U ions.canibevestablished between .the boundary and :the .anodeT-regionz...

Butzfor the icontinuect additionrof.::HC1 at: the. cathode region'the boundary would'inove toward 1 the- 'cathode and consequently theiiboundary of the general .type: herein described mas .SOIZILA times: been referred :to. as. a moving boundary. In carryingout tha present invention :the bound-.- ary is preferably restrained .from i so moving .by automatically adjusting the feed of the: acid elec trolyte into "the sytem at the :cathodei:region;

It Wil1-now=be seen that as the system continuesto operate; a separation or;:enrichment"of-the: isotope U is. established and =maintainedr'at a zone -a-just belo'w the boundary; and byriappropriate means usch as the petcock l9 'assolution of the 'uranium salt enriched as to :its U 3 "isotope content and generally 1 impoverished 1. as to its U content can 'bewithdrawn continuously ,orintermittently from this zone.

The apparatus embodying :the present inven tion-is desig ned to maintain stationarythe bound ary' between the-solutions of UOCl' and HCl;

The apparatus illustrated in its;- prefe'rred em .bodiinent" "consists of' "a resistance thermometer 1 2t immersed in an 'electrolyticbath 'withthe boundary 1 2 it located its midpoint. f The-thermometer comprises inthis embodiment of theinvention a-resistance element and may consist of "a coil 22 bifilarly wound and contained "in a 'Pyrex tube 23"." Inone specificembodiment of the "invention the resistance unit comprised an I extremely fine enameled copper wire',.andicoils of" this: unitabout -YZ icm; long and 3 in ;diameter& are contained 1" within .a r-Pyrex. tube 20f about 6 mm: in outside diameter: The. resistance or this unit". at. the. operating temperature. of. 6:59 C. is 200 OhlIlSi"; This resistance-unit :isscone nected:.preferably intoizanr arm :oi can; alternat-: of the' heavier U but.if no:.:further "stepsnrezzqs {angst-current:rWheatstone bridge 24:) The bridge-1 is brought to balance with the'boundary layer at the midsection of the coil 22 so that any change in the position of the boundary layer will change the average temperature of the thermal resistance unit thereby unbalancing the bridge. For example, a rise in the boundary layer -increases the average temperature of the resistance unit increasing the ohmic resistance of the coil 22 connected to ,form one arm. of the bridge thereby unbalancing the bridge. The output of the bridge 24 is applied to the grid of the electron tube 25 which comprises the first stage of the two stage amplifier illustrated in Fig. 2. The output from the tube 25 is applied for further amplification as an input signal to the grid of the electron tube 26 and is there combined with an adjustable voltage component which is 90 out of phase with the line voltage. This. adjustable out-of-phase voltage component is obtained across the resistor 21 and is made variable as to its magnitude by means of the adjustable tap indicated at 2B. The output of the amplifier circuit, the phase of which varies with the degree of oiT balance of the bridge, is used to give a continuous control of the plate current of the thyratron tube 29.

The use of the bridgecontrolled thyratron for temperature regulation is well known in the art and the particular circuit illustrated in Fig. 2 was patterned after a circuit devised by D. Bancroft and described in Review of Scientific Instruments, vol 13, pages 24 and 114 (1942). Accordingly, further description is omitted in the interest of clarity.

As shown in Fig. 2 the load for the plate circuit of the thyratron tube 29 comprises the heating coil 32 of the bimetallic strip 3|. As a specific example the heating coil 32 may comprise a 500 ohm constantan or nichrome wire for use with a 2050 thyratron capable of passing a maximum of 100 milliamperes. The bimetallic strip 3| is fastened at one end to a support of adjustable height and supports at its free end a leveling cup 33 provided with an overflow M. A flexible conduit 35, rubber tubing for example, communicates the liquid level of the cup 33 with a branch arm [8 of the U-shaped tube it (which comprises the electrolytic cell). On being heated by the coil 32 the strip 3! will flex altering the height of the liquid level in the cup to change the feed of acid electrolyte into the branch arm l8 and into the tube In comprising the electrolysis cell. It is preferred to utilize a bimetallic strip that will give a total throw in response to maximum current, through its heating coil, of at least 1 cm. A predetermined and constant level of the liquid electrolyte in the cup is maintained by the feed line 36 and the overflow 34. I

It may be preferred to utilize two thermal resistance elements and to connect each of these elements in opposite arms of the Wheatstone circuit for the purpose of doubling the sensitivity of the apparatus. For example, both of the resistance elements may be supported in the same Pyrex tube, each positioned in the cell with the boundary layer in the mid section of the thermal element.

It is observed that the resistance elements will respond to changes due to causes other than movement of the boundary. By locating a separate resistor in the thermometer tube, in a position above the boundary, compensation may be made for changes in the temperature of the reaction bath. Another method for compensating for changes in temperature is to utilize two separate resistance units, one mounted above and the other below the main thermionic resistance unit that is positioned at the boundary. In this Way partial compensation is obtained for temperature changes resulting from variations in the electrolysis current.

It will'be understood that the apparatus described may be used to effect separation of is'o-' topes of any metal wherein the isotopes are sufliciently di'flerent in mass to permit their segregation by the counterflows disclosed above. Furthermore, it will be obvious to those skilled in the art that electrolytes other than the ones described may be used in the separation of the particular uranium isotopes. In its broad aspect this invention involves controlling the boundary between juxtaposed solutions in an electrolytic system in response to changes in the average temperature of an element positioned within the electrolytic system in the region of said boundary. Accordingly, it is understood that other types of circuits could be employed for the same purpose, and that the particular circuit shown in detail herein is but one example of the many different ways in which the invention may be practiced.

I claim:

1. In a system including an electrically conducting fluid superimposed upon a fluid of a dif-v ferent electrical conductivity, to form a boundary therebetween, means for causing an electrolysis current to flow in said fluids to heat said fluids to a temperature inversely related to their respective conductivities, an electrical resistance unit the resistance of which varies with temperature bathed in said fluids in the region of the boundary between said fluids, a container for an additional supply of one of said fluids and means responsive to the changes in the electrical resistance of said unit, for controlling the flow of fluid from said container to said system.

2. In an electrolytic system having superimposed fluids of difierent temperatures forming a boundary therebetween apparatus for maintaining the position of the boundary which comprises a container for the additional supply of one of said fluids, a resistance unit the resistance of which varies with temperature bathed in said fluids in the region of said boundary, having an average temperature responsive to the position of said boundary, a bridge circuit including as one leg thereof the resistance of said unit and means responsive to the change in the balance of said bridge circuit resulting from the movement of said boundary relative to said unit for controlling the addition into said system of said fluid within said container.

3. In the art of titrating a liquid into a chamber containing a reaction bath, the combination comprising a chamber for said reaction bath, a leveling cup containing a supply of said liquid at a constant predetermined level therein, a bimetallic strip for supporting said cup, a control circuit responsive to changes in the electrical characteristics of said bath for heating said bimetallic strip to cause said strip to flex thereby altering the level of said cup and a flexible conduit communicating said cup and said chamber.

4. In an electrolytic system a cell containing a plurality of juxtaposed fluids of difierent electrical conductivity forming a boundary therebetween, and having electrodes disposed in said fluids on opposite sides of said boundary, for passing a predetermined electric current therethrough; means for controlling the position of I said ebmmdaryweomprising 'a' resistance: element :the resistance .ofwhich varies with 1 temperature bathed in saidyfiuidsin the :regionof saidboundary :so that 1 its average temperature varies with the -position of said" bound-ary, a supply vessel containing one of said fluids ata predetermined level therein, i-andemeans-responsive' to the e-hangesin the average temperatureof said resistance unit for altering 'the position oi -.v-said supply vessel relative to said cell for regulating thewadmission intosaid system of the fluid contained therein.

1 5: a system fonl'theelectrolytic separation 'OfiiSOtODBS of atsubstance wherein the-less mobile ions of: aheavyisotope are swept back from the terfiow which comprises .a resistance unit the resistance 110i #"which ataries with tem-perature bathed said"electrolytehavingyanaweragettemperatur-e responsi-tv'e :IEOi changes in .said counterflow and means responsive to: the changes in resistivity of said unit (for regulating the addition 0fr-an:e1ectr0lyte t0said fluid to regulate the counterflowto maintainsaid: unit atia predetermined temperature.

References. Cited the 'file "of this patent UNITED STATESIPATENTS i Name Date Longsworth tAug; '7; 1951 Number 2,563;'729 

1. IN A SYSTEM INCLUDING AN ELECTRICLLY CONDUCTING FLUID SUPERIMPOSED UPON A FLUID OF A DIFFERENT ELECTRICAL CONDUCTIVITY, TO FORM A BOUNDARY THEREBETWEN, MEANS FOR CAUSING AN ELECTROLYSIS CURRENT TO FLOW IN SAID FLUIDS TO HEAT SAID FLUIDS TO A TEMPERATURE INVERSELY RELATED TO THEIR RESPECTIVE CONDUCTIVITIES, AN ELECTRICAL RESISTANCE UNIT THE RESISTANCE OF WHICH VARIES WITH TEMPERATURE BATHED IN SAID FLUIDS IN THE REGION OF THE BOUNDARY BETWEEN SAID FLUIDS, A CONTAINER FOR AN ADDITIONAL SUPPLY OF ONE OF SAID FLUIDS AND MEANS RESPONSIVE TO THE CHANGES IN THE ELECTRICAL RESISTANCE OF SAID UNIT, FOR CONTROLLING THE FLOW OF FLUID FROM SAID CONTAINER TO SAID SYSTEM. 